Feedback effects of horizontal cell membrane potential on cone calcium currents studied with simultaneous recordings

Tandem pore domain acid-sensitive K channel 3 (TASK-3) regulates visual sensitivity in healthy and aging retina

Retinal ganglion cells (RGCs) not only collect but also integrate visual signals and send them from the retina to the brain. The mechanisms underlying the RGC integration of synaptic activity within retinal circuits have not been fully explored. Here, we identified a pronounced expression of tandem pore domain acid-sensitive potassium channel 3 (TASK-3), a two-pore domain potassium channel (K2P), in RGCs. By using a specific antagonist and TASK-3 knockout mice, we found that TASK-3 regulates the intrinsic excitability and the light sensitivity of RGCs by sensing neuronal activity-dependent extracellular acidification. In vivo, the blockade or loss of TASK-3 dampened pupillary light reflex, visual acuity, and contrast sensitivity. Furthermore, overexpressing TASK-3 specifically in RGCs using an adeno-associated virus approach restored the visual function of TASK-3 knockout mice and aged mice where the expression and function of TASK-3 were reduced. Thus, our results provide evidence that implicates a critical role of K2P in visual processing in the retina.

Origins of direction selectivity in the primate retina

From mouse to primate, there is a striking discontinuity in our current understanding of the neural coding of motion direction. In non-primate mammals, directionally selective cell types and circuits are a signature feature of the retina, situated at the earliest stage of the visual process. In primates, by contrast, direction selectivity is a hallmark of motion processing areas in visual cortex, but has not been found in the retina, despite significant effort. Here we combined functional recordings of light-evoked responses and connectomic reconstruction to identify diverse direction-selective cell types in the macaque monkey retina with distinctive physiological properties and synaptic motifs. This circuitry includes an ON-OFF ganglion cell type, a spiking, ON-OFF polyaxonal amacrine cell and the starburst amacrine cell, all of which show direction selectivity. Moreover, we discovered that macaque starburst cells possess a strong, non-GABAergic, antagonistic surround mediated by input from excitatory bipolar cells that is critical for the generation of radial motion sensitivity in these cells. Our findings open a door to investigation of a precortical circuitry that computes motion direction in the primate visual system.

Adhesion GPCR Latrophilin 3 regulates synaptic function of cone photoreceptors in a trans-synaptic manner

Cone photoreceptors mediate daylight vision in vertebrates. Changes in neurotransmitter release at cone synapses encode visual information and is subject to precise control by negative feedback from enigmatic horizontal cells. However, the mechanisms that orchestrate this modulation are poorly understood due to a virtually unknown landscape of molecular players. Here, we report a molecular player operating selectively at cone synapses that modulates effects of horizontal cells on synaptic release. Using an unbiased proteomic screen, we identified an adhesion GPCR Latrophilin3 (LPHN3) in horizontal cell dendrites that engages in transsynaptic control of cones. We detected and characterized a prominent splice isoform of LPHN3 that excludes a element with inhibitory influence on transsynaptic interactions. A gain-of-function mouse model specifically routing LPHN3 splicing to this isoform but not knockout of LPHN3 diminished Ca_V1.4 calcium channel activity profoundly disrupted synaptic release by cones and resulted in synaptic transmission deficits. These findings offer molecular insight into horizontal cell modulation on cone synaptic function and more broadly demonstrate the importance of alternative splicing in adhesion GPCRs for their physiological function.

A multiscale continuum model of the vertebrate outer retina: The temporal dynamics of background-induced flicker enhancement

The retina is a part of the central nervous system that is accessible, well documented, and studied by researchers spanning the clinical, experimental, and theoretical sciences. Here, we mathematically model the subcircuits of the outer plexiform layer of the retina on two spatial scales: that of an individual synapse and that of the scale of the receptive field (hundreds to thousands of synapses). To this end we formulate a continuum spine model (a partial differential equation system) that incorporates the horizontal cell syncytium and its numerous processes (spines) within cone pedicles. With this multiscale modeling approach, detailed biophysical mechanisms at the synaptic level are retained while scaling up to the receptive field level. As an example of its utility, the model is applied to study background-induced flicker enhancement in which the onset of a dim background enhances the center flicker response of horizontal cells. Simulation results, in comparison with flicker enhancement data for square, slit, and disk test regions, suggest that feedback mechanisms that are voltage-axis modulators of cone calcium channels (for example, ephaptic and/or pH feedback) are robust in capturing the temporal dynamics of background-induced flicker enhancement. The value and potential of this continuum spine approach is that it provides a framework for mathematically modeling the input-output properties of the entire receptive field of the outer retina while implementing the latest models for transmission mechanisms at the synaptic level.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy-H. Front Cell Neurosci 2021;15:693095. [PMID: 34539347 PMCID: PMC8446203 DOI: 10.3389/fncel.2021.693095]

There is significant evidence to support the notion that glial cells can modulate the strength of synaptic connections between nerve cells, and it has further been suggested that alterations in intracellular calcium are likely to play a key role in this process. However, the molecular mechanism(s) by which glial cells modulate neuronal signaling remains contentiously debated. Recent experiments have suggested that alterations in extracellular H+ efflux initiated by extracellular ATP may play a key role in the modulation of synaptic strength by radial glial cells in the retina and astrocytes throughout the brain. ATP-elicited alterations in H+ flux from radial glial cells were first detected from Müller cells enzymatically dissociated from the retina of tiger salamander using self-referencing H+-selective microelectrodes. The ATP-elicited alteration in H+ efflux was further found to be highly evolutionarily conserved, extending to Müller cells isolated from species as diverse as lamprey, skate, rat, mouse, monkey and human. More recently, self-referencing H+-selective electrodes have been used to detect ATP-elicited alterations in H+ efflux around individual mammalian astrocytes from the cortex and hippocampus. Tied to increases in intracellular calcium, these ATP-induced extracellular acidifications are well-positioned to be key mediators of synaptic modulation. In this article, we examine the evidence supporting H+ as a key modulator of neurotransmission, review data showing that extracellular ATP elicits an increase in H+ efflux from glial cells, and describe the potential signal transduction pathways involved in glial cell-mediated H+ efflux. We then examine the potential role that extracellular H+ released by glia might play in regulating synaptic transmission within the vertebrate retina, and then expand the focus to discuss potential roles in spreading depression, migraine, epilepsy, and alterations in brain rhythms, and suggest that alterations in extracellular H+ may be a unifying feature linking these disparate phenomena.

Transmission at rod and cone ribbon synapses in the retina

Light-evoked voltage responses of rod and cone photoreceptor cells in the vertebrate retina must be converted to a train of synaptic vesicle release events for transmission to downstream neurons. This review discusses the processes, proteins, and structures that shape this critical early step in vision, focusing on studies from salamander retina with comparisons to other experimental animals. Many mechanisms are conserved across species. In cones, glutamate release is confined to ribbon release sites although rods are also capable of release at non-ribbon sites. The role of non-ribbon release in rods remains unclear. Release from synaptic ribbons in rods and cones involves at least three vesicle pools: a readily releasable pool (RRP) matching the number of membrane-associated vesicles along the ribbon base, a ribbon reserve pool matching the number of additional vesicles on the ribbon, and an enormous cytoplasmic reserve. Vesicle release increases in parallel with Ca²⁺ channel activity. While the opening of only a few Ca²⁺ channels beneath each ribbon can trigger fusion of a single vesicle, sustained release rates in darkness are governed by the rate at which the RRP can be replenished. The number of vacant release sites, their functional status, and the rate of vesicle delivery in turn govern replenishment. Along with an overview of the mechanisms of exocytosis and endocytosis, we consider specific properties of ribbon-associated proteins and pose a number of remaining questions about this first synapse in the visual system.

Parallel Synaptic Acetylcholine Signals Facilitate Large Monopolar Cell Repolarization and Modulate Visual Behavior in Drosophila

Appropriate termination of the photoresponse in image-forming photoreceptors and downstream neurons is critical for an animal to achieve high temporal resolution. Although the cellular and molecular mechanisms of termination in image-forming photoreceptors have been extensively studied in Drosophila, the underlying mechanism of termination in their downstream large monopolar cells remains less explored. Here, we show that synaptic ACh signaling, from both amacrine cells (ACs) and L4 neurons, facilitates the rapid repolarization of L1 and L2 neurons. Intracellular recordings in female flies show that blocking synaptic ACh output from either ACs or L4 neurons leads to slow repolarization of L1 and L2 neurons. Genetic and electrophysiological studies in both male and female flies determine that L2 neurons express ACh receptors and directly receive ACh signaling. Moreover, our results demonstrate that synaptic ACh signaling from both ACs and L4 neurons simultaneously facilitates ERG termination. Finally, visual behavior studies in both male and female flies show that synaptic ACh signaling, from either ACs or L4 neurons to L2 neurons, is essential for the optomotor response of the flies in high-frequency light stimulation. Our study identifies parallel synaptic ACh signaling for repolarization of L1 and L2 neurons and demonstrates that synaptic ACh signaling facilitates L1 and L2 neuron repolarization to maintain the optomotor response of the fly on high-frequency light stimulation.SIGNIFICANCE STATEMENT The image-forming photoreceptor downstream neurons receive multiple synaptic inputs from image-forming photoreceptors and various types of interneurons. It remains largely unknown how these synaptic inputs modulate the neural activity and function of image-forming photoreceptor downstream neurons. We show that parallel synaptic ACh signaling from both amacrine cells and L4 neurons facilitates rapid repolarization of large monopolar cells in Drosophila and maintains the optomotor response of the fly on high-frequency light stimulation. This work is one of the first reports showing how parallel synaptic signaling modulates the activity of large monopolar cells and motion vision simultaneously.

Vesicular Release of GABA by Mammalian Horizontal Cells Mediates Inhibitory Output to Photoreceptors

Feedback inhibition by horizontal cells regulates rod and cone photoreceptor calcium channels that control their release of the neurotransmitter glutamate. This inhibition contributes to synaptic gain control and the formation of the center-surround antagonistic receptive fields passed on to all downstream neurons, which is important for contrast sensitivity and color opponency in vision. In contrast to the plasmalemmal GABA transporter found in non-mammalian horizontal cells, there is evidence that the mechanism by which mammalian horizontal cells inhibit photoreceptors involves the vesicular release of the inhibitory neurotransmitter GABA. Historically, inconsistent findings of GABA and its biosynthetic enzyme, L-glutamate decarboxylase (GAD) in horizontal cells, and the apparent lack of surround response block by GABAergic agents diminished support for GABA's role in feedback inhibition. However, the immunolocalization of the vesicular GABA transporter (VGAT) in the dendritic and axonal endings of horizontal cells that innervate photoreceptor terminals suggested GABA was released via vesicular exocytosis. To test the idea that GABA is released from vesicles, we localized GABA and GAD, multiple SNARE complex proteins, synaptic vesicle proteins, and Cav channels that mediate exocytosis to horizontal cell dendritic tips and axonal terminals. To address the perceived relative paucity of synaptic vesicles in horizontal cell endings, we used conical electron tomography on mouse and guinea pig retinas that revealed small, clear-core vesicles, along with a few clathrin-coated vesicles and endosomes in horizontal cell processes within photoreceptor terminals. Some small-diameter vesicles were adjacent to the plasma membrane and plasma membrane specializations. To assess vesicular release, a functional assay involving incubation of retinal slices in luminal VGAT-C antibodies demonstrated vesicles fused with the membrane in a depolarization- and calcium-dependent manner, and these labeled vesicles can fuse multiple times. Finally, targeted elimination of VGAT in horizontal cells resulted in a loss of tonic, autaptic GABA currents, and of inhibitory feedback modulation of the cone photoreceptor Cai, consistent with the elimination of GABA release from horizontal cell endings. These results in mammalian retina identify the central role of vesicular release of GABA from horizontal cells in the feedback inhibition of photoreceptors.

Horizontal Cell Feedback to Cone Photoreceptors in Mammalian Retina: Novel Insights From the GABA-pH Hybrid Model

How neurons in the eye feed signals back to photoreceptors to optimize sensitivity to patterns of light appears to be mediated by one or more unconventional mechanisms. Via these mechanisms, horizontal cells control photoreceptor synaptic gain and enhance key aspects of temporal and spatial center-surround receptive field antagonism. After the transduction of light energy into an electrical signal in photoreceptors, the next key task in visual processing is the transmission of an optimized signal to the follower neurons in the retina. For this to happen, the release of the excitatory neurotransmitter glutamate from photoreceptors is carefully regulated via horizontal cell feedback, which acts as a thermostat to keep the synaptic transmission in an optimal range during changes to light patterns and intensities. Novel findings of a recently described model that casts a classical neurotransmitter system together with ion transport mechanisms to adjust the alkaline milieu outside the synapse are reviewed. This novel inter-neuronal messaging system carries feedback signals using two separate, but interwoven regulated systems. The complex interplay between these two signaling modalities, creating synaptic modulation-at-a-distance, has obscured it’s being defined. The foundations of our understanding of the feedback mechanism from horizontal cells to photoreceptors have been long established: Horizontal cells have broad receptive fields, suitable for providing surround inhibition, their membrane potential, a function of stimulus intensity and size, regulates inhibition of photoreceptor voltage-gated Ca²⁺ channels, and strong artificial pH buffering eliminates this action. This review compares and contrasts models of how these foundations are linked, focusing on a recent report in mammals that shows tonic horizontal cell release of GABA activating Cl⁻ and HCO₃⁻ permeable GABA autoreceptors. The membrane potential of horizontal cells provides the driving force for GABAR-mediated HCO₃⁻ efflux, alkalinizing the cleft when horizontal cells are hyperpolarized by light or adding to their depolarization in darkness and contributing to cleft acidification via NHE-mediated H⁺ efflux. This model challenges interpretations of earlier studies that were considered to rule out a role for GABA in feedback to cones.

Controlling Horizontal Cell-Mediated Lateral Inhibition in Transgenic Zebrafish Retina with Chemogenetic Tools

Horizontal cells (HCs) form reciprocal synapses with rod and cone photoreceptors, an arrangement that underlies lateral inhibition in the retina. HCs send negative and positive feedback signals to photoreceptors, but how HCs initiate these signals remains unclear. Unfortunately, because HCs have no unique neurotransmitter receptors, there are no pharmacological treatments for perturbing membrane potential specifically in HCs. Here we use transgenic zebrafish whose HCs express alien receptors, enabling cell-type-specific control by cognate alien agonists. To depolarize HCs, we used the Phe-Met-Arg-Phe-amide (FMRFamide)-gated Na⁺ channel (FaNaC) activated by the invertebrate neuropeptide FMRFamide. To hyperpolarize HCs we used a pharmacologically selective actuator module (PSAM)-glycine receptor (GlyR), an engineered Cl^– selective channel activated by a synthetic agonist. Expression of FaNaC or PSAM-GlyR was restricted to HCs with the cell-type selective promoter for connexin-55.5. We assessed HC-feedback control of photoreceptor synapses in three ways. First, we measured presynaptic exocytosis from photoreceptor terminals using the fluorescent dye FM1-43. Second, we measured the electroretinogram (ERG) b-wave, a signal generated by postsynaptic responses. Third, we used Ca²⁺ imaging in retinal ganglion cells (RGCs) expressing the Ca²⁺ indicator GCaMP6. Addition of FMRFamide significantly decreased FM1-43 destaining in darkness, whereas the addition of PSAM-GlyR significantly increased it. However, both agonists decreased the light-elicited ERG b-wave and eliminated surround inhibition of the Ca²⁺ response of RGCs. Taken together, our findings show that chemogenetic tools can selectively manipulate negative feedback from HCs, providing a platform for understanding its mechanism and helping to elucidate its functional roles in visual information processing at a succession of downstream stages.

Contributions of glutamate transporters and Ca2+-activated Cl- currents to feedback from horizontal cells to cone photoreceptors

Lateral inhibitory feedback from horizontal cells (HCs) to cones establishes center-surround receptive fields and color opponency in the retina. When HCs hyperpolarize to light, inhibitory feedback to cones increases activation of cone Ca²⁺ currents (I_Ca) that can in turn activate additional currents. We recorded simultaneously from cones and HCs to analyze cone currents activated by HC feedback in salamander retina. Depolarization-activated inward tail currents in cones were inhibited by CaCCinh-A01 that inhibits both Ano1 and Ano2 Ca²⁺-activated Cl^- currents (I_Cl(Ca)). An Ano1-selective inhibitor Ani9 was less effective suggesting that Ano2 is the predominant I_Cl(Ca) subtype in cones. CaCCinh-A01 inhibited feedback currents more strongly when intracellular Ca²⁺ in cones was buffered with 0.05 mM EGTA compared to stronger buffering with 5 mM EGTA. By contrast, blocking glutamate transporter anion currents (I_Cl(Glu)) with TBOA had stronger inhibitory effects on cone feedback currents when Ca²⁺ buffering was strong. Inward feedback currents ran down at rates intermediate between rundown of glutamate release and I_Cl(Ca), consistent with contributions to feedback from both I_Cl(Ca) and I_Cl(Glu). These results suggest that Cl^- channels coupled to glutamate transporters help to speed inward feedback currents initiated by local changes in intracellular [Ca²⁺] close to synaptic ribbons of cones whereas Ano2 Ca²⁺-activated Cl^- channels contribute to slower components of feedback regulated by spatially extensive changes in intracellular [Ca²⁺].

Diverse Cell Types, Circuits, and Mechanisms for Color Vision in the Vertebrate Retina

Synaptic interactions to extract information about wavelength, and thus color, begin in the vertebrate retina with three classes of light-sensitive cells: rod photoreceptors at low light levels, multiple types of cone photoreceptors that vary in spectral sensitivity, and intrinsically photosensitive ganglion cells that contain the photopigment melanopsin. When isolated from its neighbors, a photoreceptor confounds photon flux with wavelength and so by itself provides no information about color. The retina has evolved elaborate color opponent circuitry for extracting wavelength information by comparing the activities of different photoreceptor types broadly tuned to different parts of the visible spectrum. We review studies concerning the circuit mechanisms mediating opponent interactions in a range of species, from tetrachromatic fish with diverse color opponent cell types to common dichromatic mammals where cone opponency is restricted to a subset of specialized circuits. Distinct among mammals, primates have reinvented trichromatic color vision using novel strategies to incorporate evolution of an additional photopigment gene into the foveal structure and circuitry that supports high-resolution vision. Color vision is absent at scotopic light levels when only rods are active, but rods interact with cone signals to influence color perception at mesopic light levels. Recent evidence suggests melanopsin-mediated signals, which have been identified as a substrate for setting circadian rhythms, may also influence color perception. We consider circuits that may mediate these interactions. While cone opponency is a relatively simple neural computation, it has been implemented in vertebrates by diverse neural mechanisms that are not yet fully understood.

Novel hybrid action of GABA mediates inhibitory feedback in the mammalian retina

The stream of visual information sent from photoreceptors to second-order bipolar cells is intercepted by laterally interacting horizontal cells that generate feedback to optimize and improve the efficiency of signal transmission. The mechanisms underlying the regulation of graded photoreceptor synaptic output in this nonspiking network have remained elusive. Here, we analyze with patch clamp recording the novel mechanisms by which horizontal cells control pH in the synaptic cleft to modulate photoreceptor neurotransmitter release. First, we show that mammalian horizontal cells respond to their own GABA release and that the results of this autaptic action affect cone voltage-gated Ca2+ channel (CaV channel) gating through changes in pH. As a proof-of-principle, we demonstrate that chemogenetic manipulation of horizontal cells with exogenous anion channel expression mimics GABA-mediated cone CaV channel inhibition. Activation of these GABA receptor anion channels can depolarize horizontal cells and increase cleft acidity via Na+/H+ exchanger (NHE) proton extrusion, which results in inhibition of cone CaV channels. This action is effectively counteracted when horizontal cells are sufficiently hyperpolarized by increased GABA receptor (GABAR)-mediated HCO3- efflux, alkalinizing the cleft and disinhibiting cone CaV channels. This demonstrates how hybrid actions of GABA operate in parallel to effect voltage-dependent pH changes, a novel mechanism for regulating synaptic output.

Analysis of Feedback Signaling from Horizontal Cells to Photoreceptors in Mice

Genetic manipulation of horizontal cells using a Connexin57-iCre mouse (Cx57-iCre) line combined with calcium imaging is proving to be a valuable method to study horizontal cell feedback inhibition onto photoreceptor terminals. While it is accepted that horizontal cells provide lateral inhibitory feedback to photoreceptors, the cellular mechanisms that underlie this feedback inhibition remain only partially elucidated. Feedback inhibition of photoreceptors acts via modulation of their voltage-gated calcium channels at their synaptic terminal. Calcium imaging of photoreceptors in retinal slices, therefore, reflects the impact of inhibitory feedback from horizontal cells. The development of a Cx57-iCre mouse line permits genetic manipulation of horizontal cells. In wild-type mouse retina, depolarization of horizontal cells by kainate provokes a decrease in photoreceptor Ca²⁺_i, whereas hyperpolarization by NBQX elicits an increase in photoreceptor Ca²⁺_i. These responses indicate increased feedback inhibition occurred when horizontal cells are depolarized, and decreased feedback inhibition, when hyperpolarized. This system was used to test the role of GABA release from horizontal cells in feedback inhibition by the selective elimination of VGAT/VIAAT, the inhibitory amino acid transmitter transporter that loads GABA into the synaptic vesicles of horizontal cells. Combined with calcium imaging of photoreceptors in retinal slices, the knockout of specific proteins, e.g., VGAT, provides a robust technique to test the role of GABA in feedback inhibition by horizontal cells.

Localizing Proton-Mediated Inhibitory Feedback at the Retinal Horizontal Cell-Cone Synapse with Genetically-Encoded pH Probes

Lateral inhibition in the vertebrate retina depends on a negative feedback synapse between horizontal cells (HCs) and rod and cone photoreceptors. A change in pH is thought to be the signal for negative feedback, but its spatial profile in the synaptic cleft is unknown. Here we use three different membrane proteins, each fused to the same genetically-encoded pH-sensitive Green Fluorescent Protein (GFP) (pHluorin), to probe synaptic pH in retina from transgenic zebrafish (Danio rerio) of either sex. We used the cone transducin promoter to express SynaptopHluorin (pHluorin on vesicle-associated membrane protein (VAMP2)) or CalipHluorin (pHluorin on an L-type Ca²⁺ channel) and the HC-specific connexin-55.5 promoter to express AMPApHluorin (pHluorin on an AMPA receptor). Stimulus light led to increased fluorescence of all three probes, consistent with alkalinization of the synaptic cleft. The receptive field size, sensitivity to surround illumination, and response to activation of an alien receptor expressed exclusively in HCs, are consistent with lateral inhibition as the trigger for alkalinization. However, SynaptopHluorin and AMPApHluorin, which are displaced farther from cone synaptic ribbons than CalipHluorin, reported a smaller pH change. Hence, unlike feedforward glutamatergic transmission, which spills over to allow cross talk between terminals in the cone network, the pH change underlying HC feedback is compartmentalized to individual synaptic invaginations within a cone terminal, consistent with private line communication.SIGNIFICANCE STATEMENT Lateral inhibition (LI) is a fundamental feature of information processing in sensory systems, enhancing contrast sensitivity and enabling edge discrimination. Horizontal cells (HCs) are the first cellular substrate of LI in the vertebrate retina, but the synaptic mechanisms underlying LI are not completely understood, despite decades of study. This paper makes a significant contribution to our understanding of LI, by showing that each HC-cone synapse is a "private-line" that operates independently from other HC-cone connections. Using transgenic zebrafish expressing pHluorin, a pH-sensitive GFP variant spliced onto three different protein platforms expressed either in cones or HCs we show that the feedback pH signal is constrained to individual cone terminals, and more stringently, to individual synaptic contact sites within each terminal.

Activation of retinal glial (Müller) cells by extracellular ATP induces pronounced increases in extracellular H+ flux

Small alterations in extracellular acidity are potentially important modulators of neuronal signaling within the vertebrate retina. Here we report a novel extracellular acidification mechanism mediated by glial cells in the retina. Using self-referencing H+-selective microelectrodes to measure extracellular H+ fluxes, we show that activation of retinal Müller (glial) cells of the tiger salamander by micromolar concentrations of extracellular ATP induces a pronounced extracellular H+ flux independent of bicarbonate transport. ADP, UTP and the non-hydrolyzable analog ATPγs at micromolar concentrations were also potent stimulators of extracellular H+ fluxes, but adenosine was not. The extracellular H+ fluxes induced by ATP were mimicked by the P2Y1 agonist MRS 2365 and were significantly reduced by the P2 receptor blockers suramin and PPADS, suggesting activation of P2Y receptors. Bath-applied ATP induced an intracellular rise in calcium in Müller cells; both the calcium rise and the extracellular H+ fluxes were significantly attenuated when calcium re-loading into the endoplasmic reticulum was inhibited by thapsigargin and when the PLC-IP3 signaling pathway was disrupted with 2-APB and U73122. The anion transport inhibitor DIDS also markedly reduced the ATP-induced increase in H+ flux while SITS had no effect. ATP-induced H+ fluxes were also observed from Müller cells isolated from human, rat, monkey, skate and lamprey retinae, suggesting a highly evolutionarily conserved mechanism of potential general importance. Extracellular ATP also induced significant increases in extracellular H+ flux at the level of both the outer and inner plexiform layers in retinal slices of tiger salamander which was significantly reduced by suramin and PPADS. We suggest that the novel H+ flux mediated by ATP-activation of Müller cells and of other glia as well may be a key mechanism modulating neuronal signaling in the vertebrate retina and throughout the brain.

The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell

Intrinsically photosensitive retinal ganglion cells (ipRGCs) combine direct photosensitivity through melanopsin with synaptically mediated drive from classical photoreceptors through bipolar-cell input. Here, we sought to provide a fuller description of the least understood ipRGC type, the M5 cell, and discovered a distinctive functional characteristic-chromatic opponency (ultraviolet excitatory, green inhibitory). Serial electron microscopic reconstructions revealed that M5 cells receive selective UV-opsin drive from Type 9 cone bipolar cells but also mixed cone signals from bipolar Types 6, 7, and 8. Recordings suggest that both excitation and inhibition are driven by the ON channel and that chromatic opponency results from M-cone-driven surround inhibition mediated by wide-field spiking GABAergic amacrine cells. We show that M5 cells send axons to the dLGN and are thus positioned to provide chromatic signals to visual cortex. These findings underscore that melanopsin's influence extends beyond unconscious reflex functions to encompass cortical vision, perhaps including the perception of color.

Pannexin 1 Is Critically Involved in Feedback from Horizontal Cells to Cones

Retinal horizontal cells (HCs) feed back negatively to cone photoreceptors and in that way generate the center/surround organization of bipolar cell receptive fields. The mechanism by which HCs inhibit photoreceptors is a matter of debate. General consensus exists that horizontal cell activity leads to the modulation of the cone Ca-current. This modulation has two components, one fast and the other slow. Several mechanisms for this modulation have been proposed: a fast ephaptic mechanism, and a slow pH mediated mechanism. Here we test the hypothesis that the slow negative feedback signal from HCs to cones is mediated by Panx1 channels expressed at the tips of the dendrites of horizontal cell. We generated zebrafish lacking Panx1 and found that the slow component of the feedback signal was strongly reduced in the mutants showing that Panx1 channels are a fundamental part of the negative feedback pathway from HCs to cones.

Extracellular H+ fluxes from tiger salamander Müller (glial) cells measured using self-referencing H+-selective microelectrodes

Self-referencing H⁺-selective electrodes were used to measure extracellular H⁺ fluxes from Müller (glial) cells isolated from the tiger salamander retina. A novel chamber enabled stable recordings using H⁺-selective microelectrodes in a self-referencing format using bicarbonate-based buffer solutions. A small basal H⁺ flux was observed from the end foot region of quiescent cells bathed in 24 mM bicarbonate-based solutions, and increasing extracellular potassium induced a dose-dependent increase in H⁺ flux. Barium at 6 mM also increased H⁺ flux. Potassium-induced extracellular acidifications were abolished when bicarbonate was replaced by 1 mM HEPES. The carbonic anhydrase antagonist benzolamide potentiated the potassium-induced extracellular acidification, while 300 μM DIDS, 300 μM SITS, and 30 μM S0859 significantly reduced the response. Potassium-induced extracellular acidifications persisted in solutions lacking extracellular calcium, although potassium-induced changes in intracellular calcium monitored with Oregon Green were abolished. Exchange of external sodium with choline also eliminated the potassium-induced extracellular acidification. Removal of extracellular sodium by itself induced a transient alkalinization, and replacement of sodium induced a transient acidification, both of which were blocked by 300 μM DIDS. Recordings at the apical portion of the cell showed smaller potassium-induced extracellular H⁺ fluxes, and removal of the end foot region further decreased the H⁺ flux, suggesting that the end foot was the major source of acidifications. These studies demonstrate that self-referencing H⁺-selective electrodes can be used to monitor H⁺ fluxes from retinal Müller cells in bicarbonate-based solutions and confirm the presence of a sodium-coupled bicarbonate transporter, the activity of which is largely restricted to the end foot of the cell.NEW & NOTEWORTHY The present study uses self-referencing H⁺-selective electrodes for the first time to measure H⁺ fluxes from Müller (glial) cells isolated from tiger salamander retina. These studies demonstrate bicarbonate transport as a potent regulator of extracellular levels of acidity around Müller cells and point toward a need for further studies aimed at addressing how such glial cell pH regulatory mechanisms may shape neuronal signaling.

Synaptic Ribbon Active Zones in Cone Photoreceptors Operate Independently from One Another

Cone photoreceptors depolarize in darkness to release glutamate-laden synaptic vesicles. Essential to release is the synaptic ribbon, a structure that helps organize active zones by clustering vesicles near proteins that mediate exocytosis, including voltage-gated Ca²⁺ channels. Cone terminals have many ribbon-style active zones at which second-order neurons receive input. We asked whether there are functionally significant differences in local Ca²⁺ influx among ribbons in individual cones. We combined confocal Ca²⁺ imaging to measure Ca²⁺ influx at individual ribbons and patch clamp recordings to record whole-cell I_Ca in salamander cones. We found that the voltage for half-maximal activation (V₅₀) of whole cell I_Ca in cones averaged −38.1 mV ± 3.05 mV (standard deviation [SD]), close to the cone membrane potential in darkness of ca. −40 mV. Ca²⁺ signals at individual ribbons varied in amplitude from one another and showed greater variability in V₅₀ values than whole-cell I_Ca, suggesting that Ca²⁺ signals can differ significantly among ribbons within cones. After accounting for potential sources of technical variability in measurements of Ca²⁺ signals and for contributions from cone-to-cone differences in I_Ca, we found that the variability in V₅₀ values for ribbon Ca²⁺ signals within individual cones showed a SD of 2.5 mV. Simulating local differences in Ca²⁺ channel activity at two ribbons by shifting the V₅₀ value of I_Ca by ±2.5 mV (1 SD) about the mean suggests that when the membrane depolarizes to −40 mV, two ribbons could experience differences in Ca²⁺ influx of >45%. Further evidence that local Ca²⁺ changes at ribbons can be regulated independently was obtained in experiments showing that activation of inhibitory feedback from horizontal cells (HCs) to cones in paired recordings changed both amplitude and V₅₀ of Ca²⁺ signals at individual ribbons. By varying the strength of synaptic output, differences in voltage dependence and amplitude of Ca²⁺ signals at individual ribbons shape the information transmitted from cones to downstream neurons in vision.

Kinetics of Inhibitory Feedback from Horizontal Cells to Photoreceptors: Implications for an Ephaptic Mechanism

Inhibitory feedback from horizontal cells (HCs) to cones generates center-surround receptive fields and color opponency in the retina. Mechanisms of HC feedback remain unsettled, but one hypothesis proposes that an ephaptic mechanism may alter the extracellular electrical field surrounding photoreceptor synaptic terminals, thereby altering Ca(2+) channel activity and photoreceptor output. An ephaptic voltage change produced by current flowing through open channels in the HC membrane should occur with no delay. To test for this mechanism, we measured kinetics of inhibitory feedback currents in Ambystoma tigrinum cones and rods evoked by hyperpolarizing steps applied to synaptically coupled HCs. Hyperpolarizing HCs stimulated inward feedback currents in cones that averaged 8-9 pA and exhibited a biexponential time course with time constants averaging 14-17 ms and 120-220 ms. Measurement of feedback-current kinetics was limited by three factors: (1) HC voltage-clamp speed, (2) cone voltage-clamp speed, and (3) kinetics of Ca(2+) channel activation or deactivation in the photoreceptor terminal. These factors totaled ∼4-5 ms in cones meaning that the true fast time constants for HC-to-cone feedback currents were 9-13 ms, slower than expected for ephaptic voltage changes. We also compared speed of feedback to feedforward glutamate release measured at the same cone/HC synapses and found a latency for feedback of 11-14 ms. Inhibitory feedback from HCs to rods was also significantly slower than either measurement kinetics or feedforward release. The finding that inhibitory feedback from HCs to photoreceptors involves a significant delay indicates that it is not due to previously proposed ephaptic mechanisms.

SIGNIFICANCE STATEMENT

Lateral inhibitory feedback from horizontal cells (HCs) to photoreceptors creates center-surround receptive fields and color-opponent interactions. Although underlying mechanisms remain unsettled, a longstanding hypothesis proposes that feedback is due to ephaptic voltage changes that regulate photoreceptor synaptic output by altering Ca(2+) channel activity. Ephaptic processes should occur with no delay. We measured kinetics of inhibitory feedback currents evoked in photoreceptors with voltage steps applied to synaptically coupled HCs and found that feedback is too slow to be explained by ephaptic voltage changes generated by current flowing through continuously open channels in HC membranes. By eliminating the proposed ephaptic mechanism for HC feedback regulation of photoreceptor Ca(2+) channels, our data support earlier proposals that synaptic cleft pH changes are more likely responsible.

Membrane-associated guanylate kinase scaffolds organize a horizontal cell synaptic complex restricted to invaginating contacts with photoreceptors

Synaptic processes and plasticity of synapses are mediated by large suites of proteins. In most cases, many of these proteins are tethered together by synaptic scaffold proteins. Scaffold proteins have a large number and typically a variety of protein interaction domains that allow many different proteins to be assembled into functional complexes. Because each scaffold protein has a different set of protein interaction domains and a unique set of interacting partners, the presence of synaptic scaffolds can provide insight into the molecular mechanisms that regulate synaptic processes. In studies of rabbit retina, we found SAP102 and Chapsyn110 selectively localized in the tips of B-type horizontal cell processes, where they contact cone and rod photoreceptors. We further identified some known SAP102 binding partners, kainate receptor GluR6/7 and inward rectifier potassium channel Kir2.1, closely associated with SAP102 in photoreceptor invaginations. The kainate receptor occupies a position distinct from that of the majority of AMPA receptors that dominate the horizontal cell postsynaptic response. GluR6/7 and Kir2.1 presumably are involved in synaptic processes that govern cell-to-cell communication and could both contribute in different ways to synaptic currents that mediate feedback signaling. Notably, we failed to find evidence for the presence of Cx57 or Cx59 that might be involved in ephaptic feedback signaling in this complex. The presence of SAP102 and its binding partners in both cone and rod invaginating synapses suggests that whatever mechanism is supported by this protein complex is present in both types of photoreceptors. J. Comp. Neurol. 525:850-867, 2017. © 2016 Wiley Periodicals, Inc.

Sources of protons and a role for bicarbonate in inhibitory feedback from horizontal cells to cones in Ambystoma tigrinum retina

KEY POINTS

In the vertebrate retina, photoreceptors influence the signalling of neighbouring photoreceptors through lateral-inhibitory interactions mediated by horizontal cells (HCs). These interactions create antagonistic centre-surround receptive fields important for detecting edges and generating chromatically opponent responses in colour vision. The mechanisms responsible for inhibitory feedback from HCs involve changes in synaptic cleft pH that modulate photoreceptor calcium currents. However, the sources of synaptic protons involved in feedback and the mechanisms for their removal from the cleft when HCs hyperpolarize to light remain unknown. Our results indicate that Na⁺ -H⁺ exchangers are the principal source of synaptic cleft protons involved in HC feedback but that synaptic cleft alkalization during light-evoked hyperpolarization of HCs also involves changes in bicarbonate transport across the HC membrane. In addition to delineating processes that establish lateral inhibition in the retina, these results contribute to other evidence showing the key role for pH in regulating synaptic signalling throughout the nervous system.

ABSTRACT

Lateral-inhibitory feedback from horizontal cells (HCs) to photoreceptors involves changes in synaptic cleft pH accompanying light-evoked changes in HC membrane potential. We analysed HC to cone feedback by studying surround-evoked light responses of cones and by obtaining paired whole cell recordings from cones and HCs in salamander retina. We tested three potential sources for synaptic cleft protons: (1) generation by extracellular carbonic anhydrase (CA), (2) release from acidic synaptic vesicles and (3) Na⁺ /H⁺ exchangers (NHEs). Neither antagonizing extracellular CA nor blocking loading of protons into synaptic vesicles eliminated feedback. However, feedback was eliminated when extracellular Na⁺ was replaced with choline and significantly reduced by an NHE inhibitor, cariporide. Depriving NHEs of intracellular protons by buffering HC cytosol with a pH 9.2 pipette solution eliminated feedback, whereas alkalinizing the cone cytosol did not, suggesting that HCs are a major source for protons in feedback. We also examined mechanisms for changing synaptic cleft pH in response to changes in HC membrane potential. Increasing the trans-membrane proton gradient by lowering the extracellular pH from 7.8 to 7.4 to 7.1 strengthened feedback. While maintaining constant extracellular pH with 1 mm HEPES, removal of bicarbonate abolished feedback. Elevating intracellular bicarbonate levels within HCs prevented this loss of feedback. A bicarbonate transport inhibitor, 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), also blocked feedback. Together, these results suggest that NHEs are the primary source of extracellular protons in HC feedback but that changes in cleft pH accompanying changes in HC membrane voltage also require bicarbonate flux across the HC membrane.

Dopamine D1 receptor modulation of calcium channel currents in horizontal cells of mouse retina

Horizontal cells form the first laterally interacting network of inhibitory interneurons in the retina. Dopamine released onto horizontal cells under photic and circadian control modulates horizontal cell function. Using isolated, identified horizontal cells from a connexin-57-iCre × ROSA26-tdTomato transgenic mouse line, we investigated dopaminergic modulation of calcium channel currents (ICa) with whole cell patch-clamp techniques. Dopamine (10 μM) blocked 27% of steady-state ICa, an action blunted to 9% in the presence of the L-type Ca channel blocker verapamil (50 μM). The dopamine type 1 receptor (D1R) agonist SKF38393 (20 μM) inhibited ICa by 24%. The D1R antagonist SCH23390 (20 μM) reduced dopamine and SKF38393 inhibition. Dopamine slowed ICa activation, blocking ICa by 38% early in a voltage step. Enhanced early inhibition of ICa was eliminated by applying voltage prepulses to +120 mV for 100 ms, increasing ICa by 31% and 11% for early and steady-state currents, respectively. Voltage-dependent facilitation of ICa and block of dopamine inhibition after preincubation with a Gβγ-blocking peptide suggested involvement of Gβγ proteins in the D1R-mediated modulation. When the G protein activator guanosine 5'-O-(3-thiotriphosphate) (GTPγS) was added intracellularly, ICa was smaller and showed the same slowed kinetics seen during D1R activation. With GTPγS in the pipette, additional block of ICa by dopamine was only 6%. Strong depolarizing voltage prepulses restored the GTPγS-reduced early ICa amplitude by 36% and steady-state ICa amplitude by 3%. These results suggest that dopaminergic inhibition of ICa via D1Rs is primarily mediated through the action of Gβγ proteins in horizontal cells.

Targeted Deletion of Vesicular GABA Transporter from Retinal Horizontal Cells Eliminates Feedback Modulation of Photoreceptor Calcium Channels

The cellular mechanisms underlying feedback signaling from horizontal cells to photoreceptors, which are important for the formation of receptive field surrounds of early visual neurons, remain unsettled. Mammalian horizontal cells express a complement of synaptic proteins that are necessary and sufficient for calcium-dependent exocytosis of inhibitory neurotransmitters at their contacts with photoreceptor terminals, suggesting that they are capable of releasing GABA via vesicular release. To test whether horizontal cell vesicular release is involved in feedback signaling, we perturbed inhibitory neurotransmission in these cells by targeted deletion of the vesicular GABA transporter (VGAT), the protein responsible for the uptake of inhibitory transmitter by synaptic vesicles. To manipulate horizontal cells selectively, an iCre mouse line with Cre recombinase expression controlled by connexin57 (Cx57) regulatory elements was generated. In Cx57-iCre mouse retina, only horizontal cells expressed Cre protein, and its expression occurred in all retinal regions. After crossing with a VGAT^flox/flox mouse line, VGAT was selectively eliminated from horizontal cells, which was confirmed immunohistochemically. Voltage-gated ion channel currents in horizontal cells of Cx57-VGAT^−/− mice were the same as Cx57-VGAT^+/+ controls, as were the cell responses to the ionotropic glutamate receptor agonist kainate, but the response to the GABA_A receptor agonist muscimol in Cx57-VGAT^−/− mice was larger. In contrast, the feedback inhibition of photoreceptor calcium channels, which in control animals is induced by horizontal cell depolarization, was completely absent in Cx57-VGAT^−/− mice. The results suggest that vesicular release of GABA from horizontal cells is required for feedback inhibition of photoreceptors.

Lateral Inhibition in the Vertebrate Retina: The Case of the Missing Neurotransmitter

Lateral inhibition at the first synapse in the retina is important for visual perception, enhancing image contrast, color discrimination, and light adaptation. Despite decades of research, the feedback signal from horizontal cells to photoreceptors that generates lateral inhibition remains uncertain. GABA, protons, or an ephaptic mechanism have all been suggested as the primary mediator of feedback. However, the complexity of the reciprocal cone to horizontal cell synapse has left the identity of the feedback signal an unsolved mystery.

Differential encoding of spatial information among retinal on cone bipolar cells

The retina is the first stage of visual processing. It encodes elemental features of visual scenes. Distinct cone bipolar cells provide the substrate for this to occur. They encode visual information, such as color and luminance, a principle known as parallel processing. Few studies have directly examined whether different forms of spatial information are processed in parallel among cone bipolar cells. To address this issue, we examined the spatial information encoded by mouse ON cone bipolar cells, the subpopulation excited by increments in illumination. Two types of spatial processing were identified. We found that ON cone bipolar cells with axons ramifying in the central inner plexiform layer were tuned to preferentially encode small stimuli. By contrast, ON cone bipolar cells with axons ramifying in the proximal inner plexiform layer, nearest the ganglion cell layer, were tuned to encode both small and large stimuli. This dichotomy in spatial tuning is attributable to amacrine cells providing stronger inhibition to central ON cone bipolar cells compared with proximal ON cone bipolar cells. Furthermore, background illumination altered this difference in spatial tuning. It became less pronounced in bright light, as amacrine cell-driven inhibition became pervasive among all ON cone bipolar cells. These results suggest that differential amacrine cell input determined the distinct spatial encoding properties among ON cone bipolar cells. These findings enhance the known parallel processing capacity of the retina.

Drift-diffusion simulation of the ephaptic effect in the triad synapse of the retina

Experimental evidence suggests the existence of a negative feedback pathway between horizontal cells and cone photoreceptors in the outer plexiform layer of the retina that modulates the flow of calcium ions into the synaptic terminals of cones. However, the underlying mechanism for this feedback is controversial and there are currently three competing hypotheses: the ephaptic hypothesis, the pH hypothesis, and the GABA hypothesis. The goal of this investigation is to demonstrate the ephaptic hypothesis by means of detailed numerical simulations. The drift-diffusion (Poisson-Nernst-Planck) model with membrane boundary current equations is applied to a realistic two-dimensional cross-section of the triad synapse in the goldfish retina to verify the existence of strictly electrical feedback, as predicted by the ephaptic hypothesis. The effect on electrical feedback from the behavior of the bipolar cell membrane potential is also explored. The computed steady-state cone calcium transmembrane current-voltage curves for several cases are presented and compared with experimental data on goldfish. The results provide convincing evidence that an ephaptic mechanism can produce the feedback effect seen in experiments. The model and numerical methods presented here can be applied to any neuronal circuit where dendritic spines are invaginated in presynaptic terminals or boutons.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

Calcium channels in rat horizontal cells regulate feedback inhibition of photoreceptors through an unconventional GABA- and pH-sensitive mechanism

Horizontal cells send inhibitory feedback to photoreceptors, helping form antagonistic receptive fields in the retina, but the neurotransmitter and the mechanisms underlying this signalling are not known. Since the proteins responsible for conventional Ca(2+)-dependent release of GABAergic synaptic vesicles are present in mammalian horizontal cells, we investigated this conventional mechanism as the means by which horizontal cells inhibit photoreceptors. Using Ca(2+) imaging in rat retinal slices, we confirm that horizontal cell depolarization with kainate inhibits and horizontal cell hyperpolarization with NBQX disinhibits the Ca(2+) signals produced by pH-sensitive activation of voltage-gated calcium channels (Ca channels) in photoreceptors. We show that while 100 μm Co(2+) reduces photoreceptor Ca(2+) signals, it disinhibits them at 10 μm, an effect reminiscent of earlier studies where low [Co(2+)] eliminated feedback. The low [Co(2+)] disinhibition is pH sensitive. We localized L-, N- and P/Q-type Ca channels in rat horizontal cells, and showed that both the N-type Ca channel blocker -conotoxin GVIA and the P/Q-type Ca channel blocker -agatoxin IVA increased Ca(2+) signals in photoreceptors in a pH-sensitive manner. Pronounced actions of GABAergic agents on feedback signals to photoreceptors were observed, and are pH sensitive, but are inconsistent with direct inhibition by GABA of photoreceptor [Ca(2+)]. Patch-clamp studies revealed that GABA activates a conductance having high bicarbonate permeability in isolated horizontal cells, suggesting that the commonality of pH sensitivity throughout the results could arise from a GABA autofeedback action in horizontal cells. This could change cleft pH with concomitant inhibitory influences on photoreceptor Ca channels.

Oral proton pump inhibitors disrupt horizontal cell-cone feedback and enhance visual hallucinations in macular degeneration patients

PURPOSE

Visual hallucinations (VHs) occur in macular degeneration patients with poor vision but normal cognitive function. The underlying mechanisms are poorly understood. We report the identification of pharmaceutical agents that enhance VH and use these agents to examine the contribution of retinal neurons to this syndrome.

METHODS

We detail clinical observations on VH in five macular degeneration patients treated with proton pump inhibitors having the core structure, 2-pyridyl-methylsulfinyl-benzimidazole. We tested possible retinal mechanisms using paired whole cell recordings to examine effects of these compounds on feedback interactions between horizontal cells and cones in amphibian retina.

RESULTS

Five patients with advanced wet macular degeneration described patterned VHs that were induced or enhanced by oral proton pump inhibitors. The abnormal images increased with light, disappeared in the dark, and originated in the retina, based on ophthalmodynamometry. Simultaneous paired whole cell recordings from amphibian cones and horizontal cells showed that 2-pyridyl-methylsulfinyl-benzimidazoles blocked the negative shift in voltage dependence and increase in amplitude of the calcium current (ICa) in cones that is induced by changes in horizontal cell membrane potential. These effects disrupt the negative feedback from horizontal cells to cones that is important for the formation of center-surround receptive fields in bipolar and ganglion cells, and thus for normal spatial and chromatic perception.

CONCLUSIONS

Our study suggests that changes in the output of retinal neurons caused by disturbances in outer retinal feedback mechanisms can enhance patterned visual hallucinations.

Inhibitory mechanisms that generate centre and surround properties in ON and OFF brisk-sustained ganglion cells in the rabbit retina

Lateral inhibition produces the centre-surround organization of retinal receptive fields, in which inhibition driven by the mean luminance enhances the sensitivity of ganglion cells to spatial and temporal contrast. Surround inhibition is generated in both synaptic layers; however, the synaptic mechanisms within the inner plexiform layer are not well characterized within specific classes of retinal ganglion cell. Here, we compared the synaptic circuits generating concentric centre-surround receptive fields in ON and OFF brisk-sustained ganglion cells (BSGCs) in the rabbit retina. We first characterized the synaptic inputs to the centre of ON BSGCs, for comparison with previous results from OFF BSGCs. Similar to wide-field ganglion cells, the spatial extent of the excitatory centre and inhibitory surround was larger for the ON than the OFF BSGCs. The results indicate that the surrounds of ON and OFF BSGCs are generated in both the outer and the inner plexiform layers. The inner plexiform layer surround inhibition comprised GABAergic suppression of excitatory inputs from bipolar cells. However, ON and OFF BSGCs displayed notable differences. Surround suppression of excitatory inputs was weaker in ON than OFF BSGCs, and was mediated largely by GABA(C) receptors in ON BSGCs, and by both GABA(A) and GABA(C) receptors in OFF BSGCs. Large ON pathway-mediated glycinergic inputs to ON and OFF BSGCs also showed surround suppression, while much smaller GABAergic inputs showed weak, if any, spatial tuning. Unlike OFF BSGCs, which receive strong glycinergic crossover inhibition from the ON pathway, the ON BSGCs do not receive crossover inhibition from the OFF pathway. We compare and discuss possible roles for glycinergic inhibition in the two cell types.

Distinctive patterns of alterations in proton efflux from goldfish retinal horizontal cells monitored with self-referencing H⁺-selective electrodes

The H(+) hypothesis of lateral feedback inhibition in the outer retina predicts that depolarizing agents should increase H(+) release from horizontal cells. To test this hypothesis, self-referencing H(+) -selective microelectrodes were used to measure extracellular H(+) fluxes from isolated goldfish horizontal cells. We found a more complex pattern of cellular responses than previously observed from horizontal cells of other species examined using this technique. One class of cells had an initial standing signal indicative of high extracellular H(+) adjacent to the cell membrane; challenge with glutamate, kainate or high extracellular potassium induced an extracellular alkalinization. This alkalinization was reduced by the calcium channel blockers nifedipine and cobalt. A second class of cells displayed spontaneous oscillations in extracellular H(+) that were abolished by cobalt, nifedipine and low extracellular calcium. A strong correlation between changes in intracellular calcium and extracellular proton flux was detected in experiments simultaneously monitoring intracellular calcium and extracellular H(+) . A third set of cells was characterized by a standing extracellular alkalinization which was turned into an acidic signal by cobalt. In this last set of cells, addition of glutamate or high extracellular potassium did not significantly alter the proton signal. Taken together, the response characteristics of all three sets of neurons are most parsimoniously explained by activation of a plasma membrane Ca(2+) ATPase pump, with an extracellular alkalinization resulting from exchange of intracellular calcium for extracellular H(+) . These findings argue strongly against the hypothesis that H(+) release from horizontal cells mediates lateral inhibition in the outer retina.

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Acidification of the synaptic cleft of cone photoreceptor terminal controls the amount of transmitter release, thereby forming the receptive field surround in the vertebrate retina

In the vertebrate retina, feedback from horizontal cells (HCs) to cone photoreceptors plays a key role in the formation of the center-surround receptive field of retinal cells, which induces contrast enhancement of visual images. The mechanism underlying surround inhibition is not fully understood. In this review, we discuss this issue, focusing on our recent hypothesis that acidification of the synaptic cleft of the cone photoreceptor terminal causes this inhibition by modulating the Ca channel of the terminals. We present evidence that the acidification is caused by proton excretion from HCs by a vacuolar type H(+) pump. Recent publications supporting or opposing our hypothesis are discussed.

Connexin hemichannel mediated ephaptic inhibition in the retina

Connexins are the building blocks of gap-junctions; sign conserving electrical synapses. Recently it has been shown that connexins can also function as hemichannels and can mediate a sign inverting inhibitory synaptic signal from horizontal cells to cones via an ephaptic mechanism. In this review we will discuss the critical requirements for such an ephaptic interaction and relate these to the available experimental evidence. The highly conserved morphological structure of the cone synapse together with a number of specific connexin proteins and proteoglycans present in the synaptic complex of the cones creates a synaptic environment that allows ephaptic interactions. The connexins involved are members of a special group of connexins, encoded by the GJA9 and GJA10 genes. Surprisingly, in contrast to many other vertebrates, mouse and other rodents seem to lack a GJA9 encoded connexin. The specific combination of substances that block feedback and the highly specific modification of feedback in a zebrafish lacking Cx55.5 hemichannels all point to an ephaptic feedback mechanism from horizontal cells to cones. This article is part of a Special Issue entitled Electrical Synapses.

Proton feedback mediates the cascade of color-opponent signals onto H3 horizontal cells in goldfish retina

It has been postulated that horizontal cells (HCs) send feedback signals onto cones via a proton feedback mechanism, which generates the center-surround receptive field of bipolar cells, and color-opponent signals in many non-mammalian vertebrates. Here we used a strong pH buffer, HEPES, to reduce extracellular proton concentration changes and so determine whether protons mediate color-opponent signals in goldfish H3 (triphasic) HCs. Superfusion with 10mM HEPES-fortified saline elicited depolarization of H3 HCs' dark membrane potential and enhanced hyperpolarizing responses to blue stimuli, but suppressed both depolarization by yellow and orange and hyperpolarization by red stimuli. The response components suppressed by HEPES resembled the inverse of spectral responses of H2 (biphasic) HCs. These results are consistent with the Stell-Lightfoot cascade model, in which the HEPES-suppressed component of H3 HCs was calculated using light responses recorded experimentally in H1 (monophasic) and H2 HCs. Selective suppression of long- or long-+middle-wavelength cone signals by long-wavelength background enhanced the responses to short-wavelength stimuli. These results suggest that HEPES inhibited color opponent signals in H3 HCs, in which the source of opponent-color signals is primarily a feedback from H2 HCs and partly from H1 HCs onto short-wavelength cones, probably mediated by protons.

Extracellular pH dynamics of retinal horizontal cells examined using electrochemical and fluorometric methods

Extracellular H(+) has been hypothesized to mediate feedback inhibition from horizontal cells onto vertebrate photoreceptors. According to this hypothesis, depolarization of horizontal cells should induce extracellular acidification adjacent to the cell membrane. Experiments testing this hypothesis have produced conflicting results. Studies examining carp and goldfish horizontal cells loaded with the pH-sensitive dye 5-hexadecanoylaminofluorescein (HAF) reported an extracellular acidification on depolarization by glutamate or potassium. However, investigations using H(+)-selective microelectrodes report an extracellular alkalinization on depolarization of skate and catfish horizontal cells. These studies differed in the species and extracellular pH buffer used and the presence or absence of cobalt. We used both techniques to examine H(+) changes from isolated catfish horizontal cells under identical experimental conditions (1 mM HEPES, no cobalt). HAF fluorescence indicated an acidification response to high extracellular potassium or glutamate. However, a clear extracellular alkalinization was found using H(+)-selective microelectrodes under the same conditions. Confocal microscopy revealed that HAF was not localized exclusively to the extracellular surface, but rather was detected throughout the intracellular compartment. A high degree of colocalization between HAF and the mitochondrion-specific dye MitoTracker was observed. When HAF fluorescence was monitored from optical sections from the center of a cell, glutamate produced an intracellular acidification. These results are consistent with a model in which depolarization allows calcium influx, followed by activation of a Ca(2+)/H(+) plasma membrane ATPase. Our results suggest that HAF is reporting intracellular pH changes and that depolarization of horizontal cells induces an extracellular alkalinization, which may relieve H(+)-mediated inhibition of photoreceptor synaptic transmission.

Synaptic transmission from horizontal cells to cones is impaired by loss of connexin hemichannels

In the vertebrate retina, horizontal cells generate the inhibitory surround of bipolar cells, an essential step in contrast enhancement. For the last decades, the mechanism involved in this inhibitory synaptic pathway has been a major controversy in retinal research. One hypothesis suggests that connexin hemichannels mediate this negative feedback signal; another suggests that feedback is mediated by protons. Mutant zebrafish were generated that lack connexin 55.5 hemichannels in horizontal cells. Whole cell voltage clamp recordings were made from isolated horizontal cells and cones in flat mount retinas. Light-induced feedback from horizontal cells to cones was reduced in mutants. A reduction of feedback was also found when horizontal cells were pharmacologically hyperpolarized but was absent when they were pharmacologically depolarized. Hemichannel currents in isolated horizontal cells showed a similar behavior. The hyperpolarization-induced hemichannel current was strongly reduced in the mutants while the depolarization-induced hemichannel current was not. Intracellular recordings were made from horizontal cells. Consistent with impaired feedback in the mutant, spectral opponent responses in horizontal cells were diminished in these animals. A behavioral assay revealed a lower contrast-sensitivity, illustrating the role of the horizontal cell to cone feedback pathway in contrast enhancement. Model simulations showed that the observed modifications of feedback can be accounted for by an ephaptic mechanism. A model for feedback, in which the number of connexin hemichannels is reduced to about 40%, fully predicts the specific asymmetric modification of feedback. To our knowledge, this is the first successful genetic interference in the feedback pathway from horizontal cells to cones. It provides direct evidence for an unconventional role of connexin hemichannels in the inhibitory synapse between horizontal cells and cones. This is an important step in resolving a long-standing debate about the unusual form of (ephaptic) synaptic transmission between horizontal cells and cones in the vertebrate retina.

Contrast enhancement is a fundamental feature of our visual system, initiated at the first synaptic connections in the retina. These are the synapses between photoreceptors (rods and cones) and their targets, horizontal cells and bipolar cells. Horizontal cells receive input from many cones and subsequently send a feedback signal to photoreceptors. Bipolar cells, however, receive direct input from only a few photoreceptors, but also receive indirect inhibitory input from surrounding cones via the horizontal cell feedback pathway. This organization induces the classic center/surround organization of bipolar cells and is considered the first step in contrast enhancement. Exactly how horizontal cells send feedback signals to photoreceptors has remained a mystery, however. One hypothesis posits that connexin hemichannels are involved. In this study, we tested this hypothesis using mutant zebrafish that lack connexin hemichannels specifically in horizontal cells. Our electrophysiology experiments showed that feedback is indeed reduced in these mutants, confirming that connexin hemichannels play an important role in feedback from horizontal cells to cones. In addition, we find that these mutant fish have decreased contrast sensitivity at a behavioral level, illustrating that functionally relevant contrast enhancement begins at the first synapse of the visual system.

A positive feedback synapse from retinal horizontal cells to cone photoreceptors

Cone photoreceptors and horizontal cells (HCs) have a reciprocal synapse that underlies lateral inhibition and establishes the antagonistic center-surround organization of the visual system. Cones transmit to HCs through an excitatory synapse and HCs feed back to cones through an inhibitory synapse. Here we report that HCs also transmit to cone terminals a positive feedback signal that elevates intracellular Ca²⁺ and accelerates neurotransmitter release. Positive and negative feedback are both initiated by AMPA receptors on HCs, but positive feedback appears to be mediated by a change in HC Ca²⁺, whereas negative feedback is mediated by a change in HC membrane potential. Local uncaging of AMPA receptor agonists suggests that positive feedback is spatially constrained to active HC-cone synapses, whereas the negative feedback signal spreads through HCs to affect release from surrounding cones. By locally offsetting the effects of negative feedback, positive feedback may amplify photoreceptor synaptic release without sacrificing HC-mediated contrast enhancement.

Visual images are projected by the lens of the eye onto a sheet of photoreceptor cells in the retina called rods and cones. Like the pixels in a digital camera, each photoreceptor generates an electrical response proportional to the local light intensity. Each photoreceptor then initiates a chemical signal that is transmitted to downstream neurons, ultimately reaching the brain. But unlike the pixels of a digital camera, photoreceptors indirectly inhibit one another through laterally projecting horizontal cells. Horizontal cells integrate signals from many photoreceptors and provide inhibitory feedback. This feedback is thought to underlie “lateral inhibition,” a process that sharpens our perception of contrast and color. Here we report the surprising finding that horizontal cells also provide positive feedback to photoreceptors, utilizing a mechanism distinct from negative feedback. The positive feedback signal is constrained to individual horizontal cell–photoreceptor connections, whereas the negative feedback signal spreads throughout a horizontal cell to affect many surrounding photoreceptors. By locally offsetting negative feedback, positive feedback boosts the photoreceptor signal while preserving contrast enhancement.

Horizontal cell feedback without cone type-selective inhibition mediates "red-green" color opponency in midget ganglion cells of the primate retina

The distinctive red-green dimension of human and nonhuman primate color perception arose relatively recently in the primate lineage with the appearance of separate long (L) and middle (M) wavelength-sensitive cone photoreceptor types. "Midget" ganglion cells of the retina use center-surround receptive field structure to combine L and M cone signals antagonistically and thereby establish a "red-green, color-opponent" visual pathway. However, the synaptic origin of red-green opponency is unknown, and conflicting evidence for either random or L versus M cone-selective inhibitory circuits has divergent implications for the developmental and evolutionary origins of trichromatic color vision. Here we directly measure the synaptic conductances evoked by selective L or M cone stimulation in the midget ganglion cell dendritic tree and show that L versus M cone opponency arises presynaptic to the midget cell and is transmitted entirely by modulation of an excitatory conductance. L and M cone synaptic inhibition is feedforward and thus occurs in phase with excitation for both cone types. Block of GABAergic and glycinergic receptors does not attenuate or modify L versus M cone antagonism, discounting both presynaptic and postsynaptic inhibition as sources of cone opponency. In sharp contrast, enrichment of retinal pH-buffering capacity, to attenuate negative feedback from horizontal cells that sum L and M cone inputs linearly and without selectivity, completely abolished both the midget cell surround and all chromatic opponency. Thus, red-green opponency appears to arise via outer retinal horizontal cell feedback that is not cone type selective without recourse to any inner retinal L versus M cone inhibitory pathways.

Center/surround organization of retinal bipolar cells: High correlation of fundamental responses of center and surround to sinusoidal contrasts

Receptive field organization of cone-driven bipolar cells was investigated by intracellular recording in the intact light-adapted retina of the tiger salamander (Ambystoma tigrinum). Centered spots and concentric annuli of optimum dimensions were used to selectively stimulate the receptive field center and surround with sinusoidal modulations of contrast at 3 Hz. At low contrasts, responses of both the center and surround of both ON and OFF bipolar cells were linear, showing high gain and thus contrast enhancement relative to cones. The contrast/response curves for the fundamental response, measured by a Fast Fourier Transform, reached half maximum amplitude quickly at 13% contrast followed by saturation at high contrasts. The variation of the normalized amplitude of the center and surround responses was remarkably similar, showing linear regression over the entire response range with very high correlations, r2 = 0.97 for both ON and OFF cells. The contrast/response curves of both center and surround for both ON and OFF cells were well fit (r2 = 0.98) by an equation for single-site binding. In about half the cells studied, the nonlinear waveforms of center and surround could be brought into coincidence by scaling and shifting the surround response in time. This implies that a nonlinearity, common to both center and surround, occurs after polarity inversion at the cone feedback synapse. Evidence from paired whole-cell recordings between single cones and OFF bipolar cells suggests that substantial nonlinearity is not due to transmission at the cone synapse but instead arises from intrinsic bipolar cell and network mechanisms. When sinusoidal contrast modulations were applied to the center and surround simultaneously, clear additivity was observed for small responses in both ON and OFF cells, whereas the interaction was strikingly nonadditive for large responses. The contribution of the surround was then greatly reduced, suggesting attenuation at the cone feedback synapse.

Mammalian retinal horizontal cells are unconventional GABAergic neurons

Lateral interactions at the first retinal synapse have been initially proposed to involve GABA by transporter-mediated release from horizontal cells, onto GABA(A) receptors expressed on cone photoreceptor terminals and/or bipolar cell dendrites. However, in the mammalian retina, horizontal cells do not seem to contain GABA systematically or to express membrane GABA transporters. We here report that mouse retinal horizontal cells express GAD65 and/or GAD67 mRNA, and were weakly but consistently immunostained for GAD65/67. While GABA was readily detected after intracardiac perfusion, it was lost during classical preparation for histology or electrophysiology. It could not be restored by incubation in a GABA-containing medium, confirming the absence of membrane GABA transporters in these cells. However, GABA was synthesized de novo from glutamate or glutamine, upon addition of pyridoxal 5'-phosphate, a cofactor of GAD65/67. Mouse horizontal cells are thus atypical GABAergic neurons, with no functional GABA uptake, but a glutamate and/or glutamine transport system allowing GABA synthesis, probably depending physiologically from glutamate released by photoreceptors. Our results suggest that the role of GABA in lateral inhibition may have been underestimated, at least in mammals, and that tissue pre-incubation with glutamine and pyridoxal 5'-phosphate should yield a more precise estimate of outer retinal processing.

Contrast processing by ON and OFF bipolar cells

Much of what is currently known about the visual response of retinal bipolar cells is based on studies of rod-dominant responses to flashes in the dark in the isolated retina. This minireview summarizes quantitative findings on contrast processing in the intact light-adapted retina based on intracellular recording from more than 400 cone-driven bipolar cells in the tiger salamander: 1) In the main, the contrast responses of ON and OFF cells are surprisingly similar, suggesting a need to refine the view that ON and OFF cells provide the selective substrates for processing of positive and negative contrasts, respectively. 2) Overall, the response is quite nonlinear, showing very high gain for small contrasts, some 10-15 times greater than that of cones, but then quickly approaches saturation for higher contrasts. 3) Under optimal conditions of light adaptation, both classes of bipolar cells show evidence for efficient coding with respect to the contrasts in natural images. 4) There is a marked diversity within both the ON and OFF bipolar cell populations and an absence of discrete subtypes. The dynamic ranges bracket the range of contrasts in nature. 5) For both ON and OFF cells, the receptive field organization shows a striking symmetry between center and surround for responses of the same polarity and thus opposite contrast polarities. 6) The latency difference between ON and OFF cells is about 30 ms, which seems qualitatively consistent with a delay due to the G-protein cascade in ON bipolar cells. 7) In sum, we report quantitative evidence for at least 11 transformations in signal processing that occur between the voltage response of cones and the voltage response of bipolar cells.

The effect of aminosulfonate buffers on the light responses and intracellular pH of goldfish retinal horizontal cells

Retinal horizontal cell feedback acts as a gain control at the first synapse in the visual system and generates center-surround receptive fields in the outer retina. One model of feedback proposes that elevation of protons in the photoreceptor synaptic cleft produces feedback. Most evidence supporting the proton model has depended on the effect of proton buffers, in particular aminosulfonates, but these agents could potentially have effects other than external pH regulation. We therefore determined if the effects of aminosulfonates on horizontal cell rollback, an indicator of feedback, were consistent with external proton buffering. Intracellular recording from horizontal cells in isolated goldfish retina revealed that rollback was blocked only by aminosulfonates with an acid dissociation constant suited for buffering at the pH (7.5) of the Ringer's solution. In isolated goldfish horizontal cells, aminosulfonates, even those that did not block rollback, altered intracellular pH. This suggests that the effect of aminosulfonates on rollback is not because of changing intracellular pH. Measures of both intracellular and extracellular pH revealed that treatment with either glutamate or kainate resulted in acidification. As glutamate produced both internal and external acidification, intracellular and extracellular horizontal cell pH would be expected to increase in response to light, a change consistent with the proton model of feedback.

Synaptic contribution of Ca2+-permeable and Ca2+-impermeable AMPA receptors on isolated carp retinal horizontal cells and their modulation by Zn2+

Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors are co-expressed on carp retinal horizontal cells. In the present study, we examined the synaptic contribution and Zn(2+) modulatory effect of these two AMPA receptor subtypes using whole-cell patch clamp technique. Specific Ca(2+)-permeable AMPA receptor antagonist (1-naphthyl acetyl spermine, NAS) and selective Ca(2+)-impermeable AMPA receptor blocker (pentobarbital, PB) were used to separate the glutamate-response in isolated H1 horizontal cell mediated by these two subtypes of AMPA receptors respectively. Application of 100 microM NAS substantially suppressed the current elicited by 3 mM glutamate and the remaining NAS-insensitive component was completely blocked by application of 100 microM PB. In addition, Zn(2+) had dual effects on Ca(2+)-permeable AMPA receptor-mediated current: at low concentration (10 microM), Zn(2+) potentiated the current, but at higher concentrations (100 and 1000 microM), Zn(2+) reduced the current in a dose-dependent manner. However, Zn(2+) (10, 100 and 1000 microM) failed to modulate the NAS-insensitive current mediated by Ca(2+)-impermeable AMPA receptors. Overall, our results suggest that Ca(2+)-permeable AMPA receptors contribute more to the cell's glutamate-response than Ca(2+)-impermeable AMPA receptors. Furthermore, Zn(2+) has dual effects on the Ca(2+)-permeable AMPA receptor activity without affecting Ca(2+)-impermeable AMPA receptors.

Parallel ON and OFF cone bipolar inputs establish spatially coextensive receptive field structure of blue-yellow ganglion cells in primate retina

In the primate retina the small bistratified, "blue-yellow" color-opponent ganglion cell receives parallel ON-depolarizing and OFF-hyperpolarizing inputs from short (S)-wavelength sensitive and combined long (L)- and middle (M)-wavelength sensitive cone photoreceptors, respectively. However, the synaptic pathways that create S versus LM cone-opponent receptive field structure remain controversial. Here, we show in the macaque monkey retina in vitro that at photopic light levels, when an identified rod input is excluded, the small bistratified cell displays a spatially coextensive receptive field in which the S-ON-input is in spatial, temporal, and chromatic balance with the LM-OFF-input. ON pathway block with l-AP-4, the mGluR6 receptor agonist, abolished the S-ON response but spared the LM-OFF response. The isolated LM component showed a center-surround receptive field structure consistent with an input from OFF-center, ON-surround "diffuse" cone bipolar cells. Increasing retinal buffering capacity with HEPES attenuated the LM-ON surround component, consistent with a non-GABAergic outer retina feedback mechanism for the bipolar surround. The GABAa/c receptor antagonist picrotoxin and the glycine receptor antagonist strychnine did not affect chromatic balance or the basic coextensive receptive field structure, suggesting that the LM-OFF field is not generated by an inner retinal inhibitory pathway. We conclude that the opponent S-ON and LM-OFF responses originate from the excitatory receptive field centers of S-ON and LM-OFF cone bipolar cells, and that the LM-OFF- and ON-surrounds of these parallel bipolar inputs largely cancel, explaining the small, spatially coextensive but spectrally antagonistic receptive field structure of the blue-ON ganglion cell.

Hemichannel-mediated and pH-based feedback from horizontal cells to cones in the vertebrate retina

BACKGROUND

Recent studies designed to identify the mechanism by which retinal horizontal cells communicate with cones have implicated two processes. According to one account, horizontal cell hyperpolarization induces an increase in pH within the synaptic cleft that activates the calcium current (Ca(2+)-current) in cones, enhancing transmitter release. An alternative account suggests that horizontal cell hyperpolarization increases the Ca(2+)-current to promote transmitter release through a hemichannel-mediated ephaptic mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

To distinguish between these mechanisms, we interfered with the pH regulating systems in the retina and studied the effects on the feedback responses of cones and horizontal cells. We found that the pH buffers HEPES and Tris partially inhibit feedback responses in cones and horizontal cells and lead to intracellular acidification of neurons. Application of 25 mM acetate, which does not change the extracellular pH buffer capacity, does lead to both intracellular acidification and inhibition of feedback. Because intracellular acidification is known to inhibit hemichannels, the key experiment used to test the pH hypothesis, i.e. increasing the extracellular pH buffer capacity, does not discriminate between a pH-based feedback system and a hemichannel-mediated feedback system. To test the pH hypothesis in a manner independent of artificial pH-buffer systems, we studied the effect of interfering with the endogenous pH buffer, the bicarbonate/carbonic anhydrase system. Inhibition of carbonic anhydrase allowed for large changes in pH in the synaptic cleft of bipolar cell terminals and cone terminals, but the predicted enhancement of the cone feedback responses, according to the pH-hypothesis, was not observed. These experiments thus failed to support a proton mediated feedback mechanism. The alternative hypothesis, the hemichannel-mediated ephaptic feedback mechanism, was therefore studied experimentally, and its feasibility was buttressed by means of a quantitative computer model of the cone/horizontal cell synapse.

CONCLUSION

We conclude that the data presented in this paper offers further support for physiologically relevant ephaptic interactions in the retina.

Horizontal cell feedback regulates calcium currents and intracellular calcium levels in rod photoreceptors of salamander and mouse retina

We tested whether horizontal cells (HCs) provide feedback that regulates the Ca(2+) current (I(Ca)) of rods in salamander and mouse retinas. In both species, hyperpolarizing HCs by puffing a glutamate antagonist, 6,7-dinitro-quinoxaline-2,3-dione (DNQX), onto HC processes caused a negative shift in the voltage dependence of rod I(Ca) and increased its peak amplitude. Conversely, depolarizing HCs by puffing kainic acid (KA) into the outer plexiform layer (OPL) caused a positive voltage shift and decreased rod I(Ca.) Experiments on salamander retina showed that these effects were blocked by addition of the pH buffer, Hepes. Intracellular calcium concentration ([Ca(2+)](i)) was examined in rods by confocal microscopy after loading salamander and mouse retinal slices with Fluo-4. Rods were depolarized to near the dark resting potential by bath application of high K(+) solutions. Hyperpolarizing HCs with 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo[f]quinoxaline (NBQX) enhanced high K(+)-evoked Ca(2+) increases whereas depolarizing HCs with KA inhibited Ca(2+) increases. In both species these effects of NBQX and KA were blocked by addition of Hepes. Thus, like HC feedback in cones, changes in HC membrane potential modulate rod I(Ca) thereby regulating rod [Ca(2+)](i) at physiological voltages, in both mouse and salamander retinas. By countering the reduced synaptic output that accompanies hyperpolarization of rods to light, HC feedback will subtract spatially averaged luminance levels from the responses of individual rods to local changes. The finding that HC to rod feedback is present in both amphibian and mammalian species shows that this mechanism is highly conserved across vertebrate retinas.